The present invention relates to a display device, and more particularly, to improvements in a liquid crystal display device which result in improved image quality.
An active matrix type liquid crystal display device is, for example, configured such that, on one substrate of a pair of substrates having a liquid crystal layer sandwiched therebetween, there are a plurality of scanning signal lines, a plurality of video signal lines which cross the plurality of scanning signal lines, and a plurality of pixels arranged in a matrix array. Each one of the plurality of pixels includes a switching element which is driven by a scanning signal line and a pixel electrode, to which video signals are supplied from a video signal line through the switching element. A counter electrode is formed on another substrate of the pair of substrates. The state of the light which passes through the liquid crystal layer is controlled by driving the liquid crystal using electric fields generated between the counter electrode and the pixel electrodes, thus producing a display of images.
Since the typical liquid crystal display device is not a self-luminous type display device, an auxiliary light source unit is provided for supplying light from the outside of the liquid crystal display panel. As one example, there is a known a liquid crystal display device in which a backlight is arranged on a side opposite to a display screen side (observer side) of the liquid crystal display pane, so that the liquid crystal display panel is illuminated from the back surface thereof. However, when light irradiated from the backlight leaks from a portion of a gap defined between neighboring pixel electrodes and an observer observes the leaked light, the contrast is lowered and the image quality is degraded.
Further, a parasitic capacitance is generated between the video signal line and the pixel electrode. When this parasitic capacitance is large, a phenomenon which is referred to as a vertical smear (also referred to as “vertical crosstalk”) becomes apparent and affects the image quality. This vertical smear is a phenomenon in which, when a display is performed as a white display window or a black display window, while adopting a half tone display as a background, the level of the half tone display at portions of the background at upper and lower sides (vertical direction) of the window is shifted either in the white display direction or in the black display direction, and these portions become different from portions of the background which have no window in color.
As ways to solve such a drawback, a technique is disclosed in Japanese Unexamined Patent Publication 209041/2001 (hereinafter referred to as “publication 1”) and a technique is disclosed in Japanese Unexamined Patent Publication 151699/2002 (hereinafter referred to as “publication 2”).
FIG. 15 is a diagram of a pixel portion schematically showing the constitution of the technique described in publication 1. Further, FIG. 16 is a cross-sectional view taken along a line E-E′ in FIG. 15. Here, in FIG. 15 and FIG. 16, to facilitate an understanding of the constitution of the technique disclosed in publication 1, the structure is simplified by omitting or modifying some constituent elements.
In FIG. 15, a video signal line (data line) DL has a portion thereof which overlaps with the pixel electrode PX. However, the video signal line DL has a narrow width portion, where the width is narrowed at a portion cut by the E-E′ line and the video signal line DL does not overlap with the pixel electrode PX, as shown in FIG. 16. Accordingly, it is possible to reduce the parasitic capacitance which is generated between the video signal line DL and the pixel electrode PX, which are separated by a second insulation film IN2.
However, with the provision of such a structural arrangement between the signal line DL and the pixel electrode PX, light tends to leak through a gap defined between the pixel electrode PX and the video signal line DL; and, hence, a light shielding film SLD is formed below the narrow width portion of the video signal line DL, with the film SLD being separated therefrom by way of a first insulation film IN1. By overlapping the light shielding film SLD relative to the edge portions of adjacent pixel electrodes PX, it is possible to block light which is irradiated from a backlight and is incident from a back surface of the substrate SUB1.
Here, in the technique disclosed in publication 1, the light shielding film SLD is formed of the same material as that used for forming a storage line (capacitance line) STL, which generates a storage capacitance, and the light shielding film SLD and the storage line STL are electrically insulated from each other. Further, GT indicates gate electrodes and GL indicates scanning signal lines (scanning lines).
FIG. 17 is a plan view of a pixel portion showing the constitution of the technique disclosed in publication 2. Also, in FIG. 17, to facilitate an understanding of the constitution, the structure is simplified by omitting or modifying some constituent elements. Here, constitutional elements corresponding to the constitutional elements shown in FIG. 15 are given the same numerals, and a repeated explanation thereof will be omitted.
To compare the constitution shown in FIG. 17 with the constitution shown in FIG. 15, the technique of publication 2 differs from that disclosed in publication 1 with respect to the fact that the width of the video signal line DL is fixed and with respect to the shape of the pixel electrode PX, but they are substantially equal in other respects. Since the cross-sectional view taken along a line F-F′ in FIG. 17 is the same as that of FIG. 16, an explanation thereof will be omitted.
The essential difference lies in the fact that the light shielding film SLD, which overlaps the video signal line DL, is integrally formed with the storage line STL. Accordingly, although the light shielding film SLD is in a floating state in the display device disclosed in publication 1, the light shielding film SLD has the same potential as the storage line STL in the constitution disclosed in publication 2.
However, the techniques disclosed in publication 1 and publication 2 have the following drawbacks.
In the technique of publication 1, since the light shielding film SLD is electrically floating, the display device suffers from another degradation of images that is different from vertical smear. In the display device of publication 1, since the light shielding film SLD is floating, with a change of the potential of the video signal line DL, the potential of the light shielding film SLD also will be changed. Here, however, there exists a case in which, due to the influence of static electricity or the like, out of a plurality of light shielding films SLD, the potential of only some light shielding films SLD will suddenly change without regard to the change of potential of the video signal line DL. In this case, the potential of some corresponding pixel electrodes PX will be subject to the influence of this change. As a result, this may give rise to a display having some gray scales that are remarkably different from other gray scales around the display, thus degrading the image quality of the display.
In the technique of publication 2, since the light shielding film SLD is held at the same potential as the storage line STL, the phenomenon which occurs in the display device disclosed in the publication 1 does not occur. However, the light shielding film SLD, which overlaps the video signal line DL by way of the first insulation film IN1, is held at a fixed potential that is different from the potential of the video signal line DL. As a result, the load is increased at the time of driving the display device by supplying video signals to the video signal line, the power consumption is increased, and, at the same time, the image quality is degraded due to rounding of the waveforms.